The subject invention relates to improving the electrical efficiency of amplifiers and particularly of high power amplifiers.
Many types of modern electronic equipment employ power amplifiers. More specifically, radio frequency (RF) power amplifiers are utilized in transmitters to increase the power level of amplitude modulated signals. For example wherein the magnitude of a fixed frequency carrier signal is continuously varied in accordance with the instantaneous magnitude of a lower frequency modulating signal and the carrier signal define a percentage of amplitude modulation.
A high-efficiency power amplifier (HEPA) typically comprises a variable frequency, fixed duty-cycle, switching amplifier whose modulation characteristics are derived from the waveform of the power supply voltage. That is, the output voltage of the power supply, now more appropriately called a high-level modulator, is programmed to track a variable frequency, variable amplitude reference that contains the desired modulation characteristic, namely the modulation reference.
The high-level modulator must be a switching type of regulator, as opposed to a linear regulator, otherwise the efficiency problem that the HEPA purports to solve is simply transferred to the high-level modulator. Therefore, an LC filtering scheme is required at the output of the modulator itself. It may even be desirable to derive the high-level modulator clock frequency from the HEPA in order to minimize beat frequencies due to system switching, depending upon design requirements.
The problem is that the LC filter at the output of the high-level modulator is unable to differentiate switching noise from some harmonic of the modulation reference waveform. The apparent worst case is the so-called "two-tone" test (as known to those skilled in the art), where the HEPA output consists of two signals with equal amplitudes but different RF frequencies. For such case, the modulation reference is a full-wave rectified sine wave at the difference frequency. A full-wave rectified sine wave has an unlimited frequency spectrum, due to the discontinuity in the waveform, with considerable total harmonic content. To the extent that the output filter of the high-level modulator attenuates these harmonics, tracking is not achieved.
Clearly, no physically realizable LC filtering scheme permits instantaneous tracking of a discontinuous waveform, such as a full-wave rectified sine wave. The accuracy of tracking increases with the resonant frequency of the high-level modulator output filter. This, in turn, implies a higher high-level modulator switching frequency for the same switching frequency attenuation. The higher the high-level modulation switching frequency, the greater the switching losses are and the lower the efficiency of the entire system.
Accordingly, a need exists for an improved apparatus and method for accomplishing high-efficiency power amplification.